1. Field of the Invention
The present invention relates to a thermal transfer recording apparatus for transferring an ink of an ink sheet to a recording medium to record an image on the recording medium.
The thermal transfer recording apparatuses include facsimile apparatuses, electronic typewriters, copying machines, printers, and the like.
2. Description of the Related Art
In general, a thermal transfer printer employs an ink sheet in which a heat-melt (or heat-sublimate, or the like) ink is coated on a base film. The ink sheet is selectively heated by a thermal head in correspondence with an image signal, the a melted (sublimated) ink is transferred onto recording paper or sheet to perform image recording. Since the ink sheet is one from which an ink is completely transferred to recording paper in single image recording (i.e., a so-called one-time sheet), the ink sheet must be conveyed by a distance corresponding to a recording length after a one-character or one-line image is recorded, so that a nonused portion of the ink sheet must be reliably conveyed to the next recording position. For this reason, the use of the ink sheet is increased, and the running cost of a thermal transfer printer tends to be increased as compared to a conventional thermal printer which records an image on a thermal sheet.
In order to-solve the above problem, as disclosed in U.S. Pat No. 4,456,392, Japanese Patent Laid-Open (Kokai) No. 58-201686, and Japanese Patent Publication No. 62-58917, thermal transfer printers each of which conveys recording paper and an ink sheet in the same direction with a speed difference therebetween have been proposed. As described in the above prior art, an ink sheet capable of recording a number of (n) image recording operations (multi-print sheet) is known. If this ink sheet is used, when a recording operation is continuously performed over a recording length L, a conveying length of an ink sheet which is conveyed after every image recording operation or during image recording can a length decreased to be smaller than the length L (L/n:n&gt;1). Thus, the efficiency of the ink sheet can be increased n times that of the conventional sheet, and a decrease in running cost of the thermal transfer printer can be expected. This recording system will be referred to as a multi-print system.
When the multi-print system is realized by such an ink sheet, the ink sheet must always be conveyed by a constant distance with respect to a conveying operation of a predetermined length of recording paper. In this conveying control, if the conveying operation of the ink sheet is controlled by measured rotation of a support shaft of a take-up roller of an ink sheet, the diameter of the take-up roller for taking up the ink sheet is gradually increased by the accumulated sheet. Thus, if the take-up roller is controlled to be rotated by the same amount, the conveying distance of the ink sheet at the end of the take-up operation is changed from one at the beginning of the take-up operation. For this reason, the ink sheet is clamped by a capstan roller, a pinch roller, and the like, and is conveyed upon rotation of these rollers.
However, in order to take up an ink sheet, the ink sheet must be pulled with a large force by the rollers. These rollers are distorted over long-term use, and the ink sheet may be wrinkled, thus disturbing a uniform conveying operation. In addition, these rollers require a mechanical portion complicated drive system, resulting in an increase in cost of the apparatus.
In the conventional thermal transfer printer, when a motor for conveying recording paper is rotated by one step, a platen roller is rotated to convey the recording paper by one line in a subscan direction. At the same time or immediately thereafter, when a motor for conveying an ink sheet is rotated by one step, the ink sheet is taken up by a take-up roller, and is also conveyed by one line. A thermal head is energized to perform transfer recording, so that one-line image data is recorded on recording paper. The conveying operations of the recording paper and the ink sheet and image recording processing by the thermal head are repetitively performed, thereby sequentially transferring and recording image data on the recording paper.
However, in the above-mentioned prior art, the motor for conveying the ink sheet drives the rotating shaft of the take-up roller, and is always rotated at a constant angular velocity. For this reason, when the take-up amount of the ink sheet is increased, the diameter of the take-up roller is increased. Even if the motor for conveying the ink sheet is rotated at the same angular velocity, the moving speed of the ink sheet is undesirably increased. Even when the diameter of the ink sheet take-up roller is minimal, if the transmission gears are selected with a ratio suitable; and for transmitting rotation of the conveying motor to the roll in order to convey an ink sheet by a necessary amount (e.g., by one line), as the take-up amount of the ink sheet is increased, the ink sheet is conveyed more than necessary.